Interactive effects of melt flow index and nanosilica dispersion on the mechanical and thermal properties of recycled Polypropylene/OCC wood-plastic composites

Abstract

This study investigates the combined effects of polypropylene (PP) recycling cycles (0, 1, 2, and 3 times) and nanosilica content (0, 3, and 5 wt%) on the properties of wood-plastic composites reinforced with 30 wt% Old Corrugated Container (OCC) pulp. Mechanical properties were assessed per ASTM D638, D790, and D256, while thermal and flammability characteristics were evaluated using TGA (ASTM E1131) and LOI (ASTM D2863). Results showed that increasing PP recycling cycles progressively increased MFI from 18 to 26 g/10 min (p < 0.01), confirming thermo-mechanical degradation. Tensile and flexural properties declined with repeated recycling but were significantly enhanced by 3 wt% nanosilica, with improvements of approximately 16–23% (p < 0.001); however, further increase to 5 wt% led to deterioration of about 15–18%, consistent with the dispersion-agglomeration transition. Impact strength decreased by approximately 17–23% with both recycling and nanosilica addition (p < 0.01). Nanosilica substantially improved flame retardancy, with LOI increasing by 15% (from 17.5% to 20.1%, p < 0.001), and enhanced thermal stability as evidenced by a 66% increase in TGA residual ash (from 6.29% to 10.46%). This research demonstrates that optimized nanosilica loading can partially offset the detrimental effects of polymer recycling while enhancing fire safety characteristics.